Cellular radio systems are currently in widespread use throughout the world providing telecommunications to mobile users. In order to meet the demand for transmission capacity within an available frequency band allocation, cellular radio systems divide a geographic area to be covered into a plurality of cell areas. Within each cell is positioned a base station with which a plurality of mobile stations within the cell communicate.
In general, an object of cellular radio communication system design is to have as few base stations as possible, since base stations are expensive, and require extensive effort in obtaining planning permission, and in some areas, suitable base station sites may not be available. In order to have as few base stations as possible, each base station ideally has as large a capacity as possible in order to service as large a number of mobile stations as possible. However, there are fundamental limits on the maximum user capacity of a base station for serving mobile stations because the number of mobile stations which can be served by a base station depend upon the available number of carrier frequencies, and these carrier frequencies are a limited resource in the frequency spectrum. In order to physically separate radiations at the same or closely matching frequencies, it is known to make use of directional antennas which produce directional radiation beams. The use of directional radiation beams allows greater reuse of carrier frequencies, and increases system capacity compared with omni-directional antennas or conventional sectorial antennas. Where carrier frequencies are reused between cells, the distance between cells in which a same carrier frequency is reused is planned such that co-channel interference is maintained at a tolerable level.
The type of antenna used at the base station site can potentially make significant improvements to the range and capacity of a cellular radio system. The conventional approach is to use omni-directional antennas, tri-sectored antennas or hex-sectored antennas. However, in another known approach, a base station antenna pattern comprises a beam of narrow angular widths as shown in FIGS. 1 and 2 herein. A narrow radiation beam 1 is directed by a base station Smart antenna 2 at a desired mobile station 3. The beam is narrow in both an azimuth and elevation planes, and tracks the mobile's movements. When compared to an omni-directional antenna, such a narrow beam has dual benefits of having high gain, leading to increased range in a thermal noise limited environment, and of rejecting interference from co-channel reuse cells, due to spatial separation of beams, thereby allowing higher capacity in a cell without cell splitting. A narrow beam has an advantage of reducing interference in a balanced manner on an uplink and a downlink path. On the downlink the mobile is unlikely to fall in the beams of other base station transmitters operating co-frequency re-use.
Where each cell has a number of antennas providing narrow beams which track individual mobiles, there results an overall improvement in carrier to interference (C/l) ratio due to the statistical probability that different beams re-using the same carrier frequency will be pointing in different directions, having different azimuths. The likelihood of two or more beams having a same carrier frequency intercepting each other is diminished. The narrower the beams, the lower the probability that a mobile will intercept a same frequency beam of a different cell in which the carrier frequency is re-used.
The extent of the advantage of a narrow beam antenna over an omni-directional antenna is a function of the beam width of the narrow beam antenna. The narrower the beam width, the greater the advantage. However, narrow beam antennas have increased size and complexity compared with omni-directional or tri-sectored antennas. For the purposes of this document, the use of the word "omni-directional" is intended to convey the meaning of having radiation coverage over an area corresponding to substantially the whole geographic area of a cell. Although a narrow radiation beam is formed at radio frequencies typically in the 900 MHz, 1800 MHz or 1900 MHz bands, a narrow beam can usefully be visualized as analogous to search light beams which emanate from the base station, and track the mobiles. When contrasted with an omni-directional antenna, this creates a high quality transmission path with minimal interference.
Prior art literature recognizes many of the potential benefits of narrow beam antennas.sup.1,2. The use of directional antennas in current cellular radio systems is based upon the principle of sectorization, as illustrated in FIG. 3 herein which illustrates schematically an area covered by a cell pattern in which a plurality of nominally hexagonal cells in an N=7 cluster size hexagonal pattern are each divided into three sectors, each spanning a range of 120.degree. azimuth. The main source of interference in a cellular system comes from the so called first tier reuse cells 300-305, which in the example of FIG. 3 are spaced apart from a center cell 306 by a distance of at least two intermediate cells, for example cells 307, 308. Using omni-directional antennas, the base station antenna in center cell 306 receives interference from mobile stations in all cells up to and including cells 300-305. However, if an antenna has a nominal 120.degree. beam width corresponding to a cell sectorized into three sectors (a tri-sectorized configuration) interference is received from mobile stations in only two first tier re-use cells, 303, 304. The situation can be improved by using an antenna with a 60.degree. beam width, corresponding to a hex-sectored configuration, in which case interference is received from mobiles in only one of the first tier cells. In sectorized cells, cellular radio transceivers at the base station are only connected to one sector or antenna and cannot be used in other sectors within the same cell.
Whilst provision of a relatively large number of narrow beams may help increase system capacity, the advantages provided by narrow beams are due in part to the use of smart antennas which allow steering of the beams. However, the applicants have noted that in the case of the North American Digital-AMPS cellular radio system the beams must remain spatially fixed. The reason for this is as follows:
The conventional North American digital AMPS cellular radio system comprises an analogue mobile standard, which also has provision for a digital time division multiple access (TDMA) mobile telephony standard compatible with the analogue mobile standard. The North American digital AMPS is also referred to as International Standard IS-54, which is an analogue/digital dual mode standard operating at 850 MHz band and more recently superseded by International Standard IS-136 dual mode analogue/digital mobile standard encompassing operation at 850 MHz band and 1900 MHz band. The D-AMPS standard includes a constraint on the downlink which precludes the use of using individually steered beams for each subscriber mobile station.
In the D-AMPS standard, a set of frequency slots each 30 kHz wide are provided, each of which can be used as an analogue channel, or as a time division multiple access digital channel. In the part of the D-AMPS standard relating to digital operation, each carrier frequency at the center of a respective 30 kHz frequency band, is used to carry a plurality of time division multiple access digital channels. Currently, operators divide each carrier frequency into three digital channels on a TDMA basis. This gives three time slots per digital frame, which has a duration of around 20 ms as illustrated in FIG. 4 herein. The D-AMPS standard specifies that whenever any one carrier has a time slot in use, i. e. a single channel to a single subscriber mobile station, then the radiation beam comprising that carrier frequency must be maintained to the subscriber mobile during not only the time slot carrying the channel communicating with that mobile subscriber, but also for the duration of the other time slots in the same frame. As soon as a carrier frequency is assigned to one subscriber digital mobile, the radiation beam cannot be moved off that mobile for other time slots within the same frame. This precludes the use of beam shifting between mobiles on a time slot by time slot basis because the subscriber mobile apparatus operating in D-AMPS receives a 20 bit synchronization sequence during its own time slot in the frame, and additionally may also look in the 20 bit synchronization sequence of preceding and succeeding time slots. Referring to FIG. 5 herein, operation of the D-AMPS handset is illustrated with reference to three time slots, representing three different channels in a D-AMPS transmission. A subscriber mobile handset allocated to a second time slot 500 within digital frame 501 inspects a synchronization training sequence 502 within its own time slot 500, as well as receiving synchronization sequences 503, 504 of respective corresponding time slots 505, 506 corresponding to other channels in the digital frame. Thus, if the other time slots 505, 506 in the digital frame 501 are effectively removed from the subscriber mobile by moving the radiation beam away from the subscriber mobile during those other time slots, the mobile cannot read the synchronization sequences in those other time slots and would experience degraded performance.
Thus, in the D-AMPS system, the statistical advantage provided by the technique of beam steering of a large number of narrow beams using smart antennas is not available and it is not possible to devise a scheme which allows beams to be dynamically moved from one mobile direction to another from one time slot to the next. Therefore, a downlink beam former for D-AMPS has to use a fixed beam approach, where the optimum beam is selected by uplink measurements of signal quality.
As a solution to increasing the capacity of the D-AMPS cellular radio system, the inventors have proposed the use of a sector covered by a plurality of non-orthogonal overlapping beams which are spatially fixed. This proposal has the advantage of increasing the number of directional beams in a sector, thereby allowing an increase in the frequency reuse, and consequent increase in capacity whilst minimizing the need for hand-over between beams which would be otherwise incurred with an equivalent number of orthogonal.sup.3 narrow overlapping beams. However, the task of producing a set of non-orthogonal overlapping beams introduces its own problems in terms of how to achieve such a set of beams from a relatively small antenna aperture, and how to generate the beams in a power efficient manner, as will become apparent from the following detailed description of the best mode herein.
In this specification, the term orthogonal, when used in relation to beams is to be construed as having a meaning as indicated in the prior art, as exemplified in "Introduction to Antennas" by Martin S Smith.sup.3, chapter 6, and the term non-orthogonal is to be construed accordingly.